Report of Key Comparison SIM.QM-K91

Transcription

1 Report of Key Comparson SIM.QM-K91 ph of phthalate buffer (nomnal ph ~4.01 at 25 ºC) Fnal Report F.B. Gonzaga, J.C. Das, K.W. Pratt, J. Waters, L. Dmtrova, M. Delgado, G.T. Canaza, R.O. Crstancho, L.A. Chavarro, S. Fajardo, P. Rodruangthum, N. Tangpasarnkul, V. Gavrlkn, S. Nagbn, A. Petrenko, A. Manska May 2015

2 Abstract At the SIM meetng n Buenos Ares, from 30 May to 1 June 2012, t was decded to perform a RMO Key comparson on ph measurement coordnated by INMETRO. Therefore, ths RMO Key comparson, named SIM.QM-K91, amed to nvestgate the degree of equvalence of measurement procedures for the ph determnaton of a phthalate buffer soluton (nomnal ph around 4.01 at 25 C). Phthalate buffer s wdely used to calbrate ph electrodes n the acd range. A buffer soluton of 0.05 mol kg 1 potassum hydrogen phthalate, KHC 8 H 4 O 4, s one of the prmary ph reference buffer solutons recommended by IUPAC [1]. It was only allowed to partcpate n ths comparson by usng a dfferental cell [2] or a glass electrode, nstead of prmary cells [1], f the hghest metrologcal standard n the nsttute or f the CMCs are based on the type of cell to be used. The results obtaned by INMETRO and NIST (whch have also partcpated n CCQM-K91 comparson) were used to lnk the results from the other nsttutes to the KCRV of CCQM-K91. In ths comparson, ph measurements were performed at 25 C, and optonally also at 15 C and 37 C. Nne nsttutes took part of the comparson, whose results are presented n ths report. 2

3 Contents ABSTRACT... 2 INTRODUCTION... 4 METROLOGY AREA... 4 BRANCH... 4 SUBJECT... 4 TIME SCHEDULE... 4 COORDINATING LABORATORY... 4 PARTICIPANTS... 4 SAMPLE DESCRIPTION... 5 SAMPLE PREPARATION AND DISTRIBUTION... 5 CHECK OF BOTTLES INTEGRITY... 5 CHECK OF HOMOGENEITY AND SHORT TERM STABILITY... 6 CHECK OF LONG TERM STABILITY... 7 RESULTS AND DISCUSSION... 8 REPORTED RESULTS... 8 DEGREES OF EQUIVALENCE AND LINK TO CCQM-K HOW FAR E LIGHT SHINES ACKNOWLEDGMENTS REFERENCES

4 Introducton Metrology area Amount of Substance Branch Electrochemstry Subject Determnaton of the acdty functon at zero chlorde molalty (pa 0 ) of an unknown phthalate buffer (nomnal ph ~4.01 at 25 C) at 25 C, and optonally at 15 C and 37 C, by usng Harned cell, dfferental potentometrc cell, or glass electrode measurements. Tme schedule Dspatch of the samples 01 October 2014 Deadlne for recept of the measurement report 30 November 2014 Draft A Report 22 January 2015 Draft B Report 02 Aprl 2015 Coordnatng laboratory Insttuto Naconal de Metrologa, Qualdade e Tecnologa (INMETRO) Av. Nossa Senhora das Graças, 50, Duque de Caxas, RJ, Brazl Fabano Barber Gonzaga Tel: Fax: Emal: fbgonzaga@nmetro.gov.br Partcpants Nne nsttutes, ncludng INMETRO, have partcpated n the SIM.QM-K91 comparson. Informaton about these nsttutes are shown n Table 1. Table 1. Partcpants n the SIM.QM-K91 comparson. N o Country Insttute Acronym Contact Person 1 BG Bulgaran Insttute of Metrology BIM L. Dmtrova 2 BO Insttuto Bolvano de Metrología IBMETRO M. Delgado 3 BR Insttuto Naconal de Metrologa, Qualdade e Tecnologa INMETRO F.B. Gonzaga 4

5 4 CO 5 Insttuto Naconal de Metrología de Colomba Insttuto Naconal de Defensa de la Competenca y de la Proteccon de la Propedad Intelectual INM R.O. Crstancho INDECOPI G.T. Canaza 6 Natonal Insttute of Metrology NIMT N. Tangpasarnkul 7 Ukrmetrteststandart UMTS V. Gavrlkn 8 US 9 UY Natonal Insttute of Standards and Technology Laboratoro Tecnológco del Uruguay NIST LATU K.W. Pratt S. Fajardo Sample descrpton Sample preparaton and dstrbuton On 21 March 2014 one batch of phthalate buffer soluton was prepared from deonzed water and potassum hydrogen phthalate and transferred to 120 HD bottles (250 ml each). The bottles were properly closed, labeled and numbered (accordng to the order fllng), sealed usng paraffn tape and put nto plastc bags. The mass fracton of water n the soluton was w(h2o) = g g 1. Each partcpant receved from four to eght HD bottles, selected randomly. Shpment to all partcpants was performed on 1 October 2014 by courer company FedEx and the trackng number was reported by e-mal. Except IBMETRO, all partcpants receved the samples up to 13 October IBMETRO has only receved the samples on 11 November Check of bottles ntegrty The partcpants were requested to nspect the receved bottles for vsble damage (bottle broken or wth leakage) and to wegh them n order to verfy f they keep unchanged durng the transport. No vsble damage was reported for all bottles. Fgure 1 shows the relatve devaton for the bottles, taken nto account the mass at orgn (measured at INMETRO) and the mass at destnaton (reported by the other nsttutes). Although IBMETRO has receved eght bottles, the masses of only three bottles (used n the measurements) were reported. As can be seen, all bottle masses reported agreed wthn 0.01% wth the bottle masses measured at INMETRO. 5

6 Relatve Devaton (%) Bottle Number Fgure 1. Relatve devaton between the masses at orgn (INMETRO) and destnaton. Check of homogenety and short term stablty Before shpment, the homogenety and the short term stablty of the samples were checked by dfferental potentometrc measurements, at 25 C, usng a Baucke-based cell [2]. For the homogenety, one bottle from the begnnng of the batch (consderng the fllng order) was measured aganst four other bottles along the batch. The data are shown n Table 2. As can be seen, all potental dfferences measured are related to ph devatons (between the reference and testng bottles) lower than , ndcatng that there was no sgnfcant ph dfferences between the bottles. Table 2. Results obtaned for the homogenety study. Reference Bottle Testng Bottle ΔE (µv) ΔpH For the short term stablty, two reference bottles stored at room temperature (22 C) were measured aganst four other bottles stored at 4 and 50 C for 15 and 30 days. The bottles were selected randomly. The data are shown n Table 3. As can be seen, although the potental dfferences measured were slghtly hgher than those obtaned n the homogenety study, the values are stll related to neglgble ph devatons (between the reference and testng bottles), ndcatng that there was no sgnfcant ph varatons when the samples were stored at 4 or 50 C for up to 30 days. 6

7 Reference Bottle Table 3. Results obtaned for the short term stablty study. Temp. Reference ( C) 22 Testng Bottle Temp. Testng ( C) Storage Tme (days) ΔE (µv) ΔpH Check of long term stablty In order to check the stablty of the sample, prmary ph measurements were taken at 25 C for some bottles, selected randomly, n rregular ntervals along about four months (from 16 September 2014 to 15 January 2015). The measurement results are gven n Fgure 2. For these results, the lnear regresson statstcal test (usng the least-squares method), for 95% confdence level, gave a p-value (for the tme) of 0.979, showng that the acdty functon varaton over tme was statstcally not sgnfcant (p-value > 0.05). In addton, the chsquare statstcal test, also for 95% confdence level, gave a calculated ch-square of 2.546, whch s lower than the tabled ch-square (9.488), showng that the acdty functon results over tme were statstcally smlar, takng nto account ther uncertantes pa Sep 23 Sep 7 Oct 21 Oct 4 Nov Date Fgure 2. Measurement results for checkng the stablty of the samples, wth expanded uncertantes (k=2). 18 Nov 2 Dec 16 Dec 30 Dec 13 Jan 7

8 Results and dscusson Reported results All nsttutes have sent ther measurement reports. The tmetable and some measurement detals used by the nsttutes are gven n Table 4. IBMETRO and INDECOPI dd not nform n ther measurement reports the tme perods of when ther measurements were taken. The results reported by the nsttutes are shown n Tables 5 to 7 and Fgures 3 to 5. The ph values reported by those nsttutes that used a dfferental potentometrc cell were recalculated as pa 0 by usng a buffer onc strength of mol kg 1 and Debye Hückel temperature-dependent constants cted n the lterature [1]. Country Sample Receved Table 4. Tmetable and some measurement detals. Measurement Perod Report Date Measurement Cell Measurement Temperatures BG 9 Oct 21 to 25 Nov 30 Nov Prmary 15, 25, 37 C BO 11 Nov 28 Nov Dfferental (INMETRO CRM) 25 C BR 13 to 26 Nov 28 Nov Prmary 15, 25, 37 C CO 6 Oct 16 to 23 Oct 18 Nov 13 Oct 1 Dec Dfferental (BIM CRM) Dfferental (NIST and PTB CRMs) 25 C 15, 25, 37 C 6 Oct 8 Dec 19 Dec Prmary 15, 25, 37 C 9 Oct 13 to 18 Nov 29 Nov Prmary 15, 25, 37 C US 3 Oct 6 to 11 Nov 28 Nov Prmary 15, 25, 37 C UY 9 Oct 18 to 25 Nov 28 Nov Dfferental (NIST CRM) 15, 25, 37 C Table 5. Results reported by the nsttutes, at 15 C, wth standard uncertantes (u). Country ph pa 0 u (k=1) BG BR US UY

9 Table 6. Results reported by the nsttutes, at 25 C, wth standard uncertantes (u). Country ph pa 0 u (k=1) BG BO BR CO US UY Table 7. Results reported by the nsttutes, at 37 C, wth standard uncertantes (u). Country ph pa 0 u (k=1) BG BR US UY o C BR BG UY US pa 0 Fgure 3. Acdty functon results, at 15 C, wth standard uncertantes (k=1). 9

10 o C pa BO BR BG UY CO US Fgure 4. Acdty functon results, at 25 C, wth standard uncertantes (k=1). pa o C BG UY BR US Fgure 5. Acdty functon results, at 37 C, wth standard uncertantes (k=1). Degrees of equvalence and lnk to CCQM-K91 In order to lnk the results of the present key comparson to those of the orgnal comparson CCQM-K91 [3], the results of INMETRO and NIST (lnkng laboratores) n both comparsons were taken nto account n the calculaton of the degrees of equvalence (D ) for the other nsttutes n the present comparson, as shown prevously [4,5]. For that, the average degree of equvalence of INMETRO and NIST n the orgnal comparson ( D calculated accordng to Equaton 1. avg-ccqm ) was D avg-ccqm AF avg-ccqm KCRV CCQM (1) Where A F avg- CCQM s the average result from INMETRO and NIST n the orgnal comparson and KCRV CCQM s the reference value of the orgnal comparson. Therefore, the D avg-ccqm 10

11 value was used n the calculaton of the degrees of equvalence for the other nsttutes n the present comparson ( D ), as shown n Equaton 2. D AF AF avg-ccqavg-sim D (2) Where A F s the result of a gven nsttute n the present comparson and A F avg- SIM s the average result from INMETRO and NIST also n the present comparson. For the calculaton of the degree of equvalence uncertantes for the other nsttutes n the present comparson ( U D ), the standard uncertanty of the KCRV value of the orgnal comparson ( u KCRV ) CCQM was also taken nto account, as shown n Equaton 3. U D 2 u 2 AF u 2 AF avg-sim u 2 KCRV CCQM (3) Where u AF s the standard uncertanty of a gven nsttute n the present comparson and u AF avg-sim s the standard uncertanty of the average result between INMETRO and NIST also n the present comparson. The u AF avg-sim was calculated accordng to Equaton 4. u AF avg-sim 2 s N Where s s the standard devaton between the results of INMETRO and NIST n the present comparson and N s the number of results (equals to 2 n ths case). The D and U results are gven n Table 8. Fgures 6 to 8 show plots of these results n comparson to those of the orgnal comparson. Table 8. Degrees of equvalence lnked to CCQM-K91, wth expanded uncertantes (U). Country (4) 15 C 25 C 37 C D U (k=2) D U (k=2) D U (k=2) BG BO CO UY D 11

12 o C D CCQM-K91 SIM.QM-K MX BG rev BR FR HU PL JP RU DK SK DE US BG UY Fgure 6. Degrees of equvalence lnked to CCQM-K91, wth expanded uncertantes (k=2), for results at 15 C o C D CCQM-K91 SIM.QM-K MX BG rev BR FR PL JP DK DE RU US SK HU TR BO BG UY CO Fgure 7. Degrees of equvalence lnked to CCQM-K91, wth expanded uncertantes (k=2), for results at 25 C. 12

13 D o C CCQM-K91 SIM.QM-K91 TR HU FR PL BR BG rev MX RU DK JP US DE SK BG UY Fgure 8. Degrees of equvalence lnked to CCQM-K91, wth expanded uncertantes (k=2), for results at 37 C. How far the lght shnes The results can be consdered to be representatve for measurement capabltes n the range from ph 4.0 to 4.2 (at 25 C). Acknowledgments INMETRO gratefully acknowledges the contrbutons of all partcpants and of the members of the CCQM Workng Group on Electrochemcal Analyss for ther valuable suggestons concernng the measurement protocol and the evaluaton process. References 1. K.W. Pratt, W.F. Koch, Y.C. Wu, P.A. Berezansky, Pure Appl. Chem. 73 (2001) F.G.K. Baucke, J. Electroanal. Chem. 368 (1994) Fnal report on CCQM-K91, Physkalsch-Technsche Bundesanstalt, 2013 (avalable at 4. CCQM Gudance note: Estmaton of a consensus KCRV and assocated Degrees of Equvalence, Consultatve Commttee for Amount of Substance, Verson 10, Fnal report on CCQM-K19.1, Physkalsch-Technsche Bundesanstalt, 2012 (avalable at 13